In order to further understand the processes involved in transepithelial ion transport, studies must be performed which allow for the differentiation between events taking place at the apical cell membrane and those ocurring at the basolateral membrane. This is especially true since it now seems that Na transport across the apical membrane is not simply diffusional. This strongly indicates that Na regulation by the apical membrane is the major factor in the control of transepithelial Na transport. We propose to study, using electrophysiological methods, the properties of the individual apical and basolateral cell membranes of the Necturus urinary bladders. Our studies will include both measurements using voltage clamp techniques and ones that will not. The nonvoltage clamp experiments will involve the investigation of intercellular communication via gap junctions and the determination of the space constant of the tissue under various conditions. The voltage clamping of the membranes on one side of the tissue, either the apical or basolateral, will be performed so that we can determine current-voltage relations and carry out current fluctuation (noise) analysis. Independent current-voltage measurements of the apical and basolateral membranes will be conducted with various solutions.